Emulsion or solution polymers are used to saturate paper substrates to impart tensile and stiffness properties, humidity resistance, and durability under high temperature conditions. Filter paper production is one of the major application areas in which paper saturation is utilized. Currently, two different polymer systems are typically used in the industry to saturate paper, depending upon whether the end use for the paper is an air filter or an oil filter. For air filters, water-based emulsion polymers, such as ethylene-co-vinyl chloride emulsions together with an external crosslinker, such as a melamine-formaldehyde resin, are typically used as paper saturants. Using these water-based emulsion polymers, an air filter paper is produced which displays adequate air permeability, wet tensile strength, and wet stiffness. However, with advances in technologies, such as heavy duty machinery, farm equipment, heavy duty trucks, and advanced internal combustion engine designs, demands have increased for improvement of water resistance, solvent and oil resistance, and high temperature stiffness of filter papers. In addition, environmental concerns impose other constraints and limitations on what is considered an acceptable binder system. For example, systems with high formaldehyde levels resulting from the use of melamine-formaldehyde crosslinkers can be problematic.
For higher performance filter papers such as hot oil filters, fuel filters and vacuum filters, solvent-based phenolic resin binders have commonly been used. However, there are disadvantages in using solvent-based phenolics, such as environmental concerns related to solvent VOC""s, problems with handling residual phenol, and environmental concerns regarding residual formaldehyde. Due, in part, to the disadvantages associated with solvent-based resin binders, there is a desire in the industry to switch to all water-based binder systems which have comparable performance to the phenolic resins for high performance filter papers.
Some currently known water-based binder systems for filters are described below:
U.S. Pat. No. 4,623,462 (Urig et al., 1986) discloses oil filters containing water-based latex binders. A filter substrate is impregnated with a water-based binder comprising a latex containing at least 20% polymerized vinyl chloride. The latex is a copolymer of vinyl chloride, 30 to 60% lower alkyl acrylate, and up to 5% one or more comonomers selected from acrylic acid and N-methylol acrylamide, based on the weight of monomers used to make the latex; said binder also containing 5 to 20 parts of a crosslinking resin per 100 weight parts of latex solids and 5 to 20% catalyst for the crosslinking resin based on the weight of the crosslinking resin.
U.S. Pat. No. 4,999,239 (lacoviello et al., 1991) discloses a binder composition, for application onto non bonded filter paper. The binder composition is an aqueous emulsion containing an ethylene-vinyl chloride copolymer and tetramethylol glycoluril and are prepared by incorporating 4 to 10 wt % tetramethylol glycoluril, based on the total weight of the emulsion copolymer, into an aqueous dispersion of a copolymer consisting essentially of 65 to 90 wt % vinyl chloride, 5 to 35 wt % ethylene and optionally, up to 10 wt % of a hydroxyalkyl- or carboxylic acid-containing functional comonomer. The polymerization is conducted in the presence of a poly(vinyl alcohol) stabilizing system. Filter paper, formed by impregnating paper or other suitable substrate with the copolymer emulsion, is reported to show enhanced resistance to hot oil.
U.S. Pat. No. 5,244,695 (Davidovich et al., 1993) discloses a method for making a nonwoven filter in which a nonwoven filter substrate is impregnated with a curable binder composition, consisting essentially of 10 to 100 wt % a poly(vinyl alcohol) which is at least 98 mol % hydrolyzed and has a DPn of 100 to 2300; and 0 to 90 wt % of an aqueous polymer emulsion. The aqueous polymer emulsion is an ethylene-vinyl chloride copolymer emulsion or a vinyl acetate/N-methylolacrylamide copolymer emulsion, or both.
U.S. Pat. No. 5,354,803 (Dragner et al., 1994) discloses a poly(vinyl alcohol) graft copolymer binder emulsion for nonwoven products. The binder emulsion comprises 12 to 35% of a fully hydrolyzed low or ultra low molecular weight poly(vinyl alcohol) (at least 75% of the poly(vinyl alcohol) is at least 98 mol % hydrolyzed) and 65 to 88% of a vinyl and/or acrylic monomer which is graft copolymerized with the poly(vinyl alcohol). Examples of suitable vinyl and acrylic monomers include C1 to C8 acrylate and methacrylate esters, vinyl acetate, styrene, acrylic acid, and methacrylic acid.
This invention is directed to high performance filters which are formed from a substrate or basestock, such as a nonwoven or a cellulosic material, impregnated with an aqueous based poly(vinyl alcohol) graft emulsion copolymer as binder.
The aqueous based poly(vinyl alcohol) graft emulsion copolymers are prepared by graft polymerizing poly(vinyl alcohol) with one or more ethylenically unsaturated monomer, and optionally, a crosslinking monomer, in an aqueous medium, in which 30 to 100% of the total poly(vinyl alcohol) is 70 to 97 mol % hydrolyzed.
The aqueous based graft emulsion copolymer of this invention can be combined with a crosslinking agent and/or catalyst and applied to a filter basestock, such as filter paper. The treated basestock can then dried at an elevated temperature to produce a filter media which is suitable for a variety of high performance applications, such as air filters, hot oil filters, and vacuum filters.
There are several advantages in using the above described poly(vinyl alcohol) graft emulsion copolymers as binders to produce high performance filters. For example:
no solvent VOC""s or residual formaldehyde are released into the environment during the production of the aqueous graft emulsion copolymer;
only a single curing cycle is required after the graft emulsion copolymer is applied to the filter basestock;
the treated basestock exhibits an excellent balance of stiffness, tensile, and air permeability; and
the treated basestock can be used in a wide range of filter grades, especially high performance filters, such as air, hot oil, fuel, and vacuum filters.
The aqueous grafted copolymer emulsions of this invention can be prepared by free radical initiated polymerization of one or more ethylenically unsaturated monomer in an aqueous solution of poly(vinyl alcohol).
Poly(vinyl alcohol) is derived from vinyl acetate polymers. The vinyl acetate polymers are formed by polymerizing vinyl acetate as a homopolymer or in conjunction with other monomers to form copolymers and are then hydrolyzed to form poly(vinyl alcohol) or vinyl alcohol copolymers. The mol percent of vinyl alcohol in the poly(vinyl alcohol) or vinyl alcohol copolymers should be sufficient to enable free radical graft polymerization in an aqueous solution; i.e., render the polymer at least partially soluble in an aqueous medium.
The poly(vinyl alcohol) used in this invention, generally, has a weight average molecular weight (MW) ranging from about 5,000 to 150,000, preferably 10,000 to 120,000. Alternatively, the poly(vinyl alcohol) can have a degree of polymerization (Dp) of from 100 to 3,000, preferably 100 to 2000. Poly(vinyl alcohol) is made commercially by the hydrolysis of poly(vinyl acetate) and typically has a hydrolysis level ranging from about 85 to greater than 99 mol %. For this invention, the level of hydrolysis can range from 70 to greater than 99 mol %, preferably 85 to 99 mol %, provided that at least 30% of the total poly(vinyl alcohol) used in the grafting reaction is 70 to 97 mol % hydrolyzed, preferably 85 to 95 mol % hydrolyzed. Mixed poly(vinyl alcohol) grades, from combinations of poly(vinyl alcohol) polymers which vary in molecular weight and hydrolysis level, can be employed in the present invention.
Suitable monomers which can be employed for graft copolymerization onto poly(vinyl alcohol) to prepare the poly(vinyl alcohol) graft copolymer emulsion include, but are not limited to, vinyl acetate, styrene, vinyl chloride, C1 to C,12 alkyl acrylates and C1 to C12 alkyl methacrylates, such as ethyl methacrylate, methyl methacrylate, 2-ethylhexyl acrylate, butyl acrylate, propyl acrylate, ethyl acrylate, methyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, and mixtures thereof. Preferred monomers are vinyl acetate, styrene, vinyl chloride, and mixtures thereof.
Other monomers, such as crosslinking monomers, which may be present include, acrylamide, methacrylamide, N-methylolacrylamide, acetoacetoxyethyl methacrylate, maleic acid, and alkyl and dialkyl maleate estersi wherein alkyl is C1 to C12.
The free radical graft copolymerization reactions can be conducted in aqueous media at a temperature necessary to liberate free radicals for the graft polymerization. Typical temperatures range from 50 to 95xc2x0 C., preferably 60 and 90xc2x0 C.
Total reaction solids levels can vary from 20 to 60 wt %, depending on the molecular weight of the poly(vinyl alcohol), preferably from 30 to 55 wt %.
Reaction times typically range from 1 to 10 hours. The graft copolymerization reaction is preferably conducted in a manner in which the total free monomer concentration in the reaction mixture is minimized to enhance its graft polymerization with poly(vinyl alcohol) instead of simple homopolymerization with itself. Total free monomer concentrations can be less than 5%, preferably less than 2%.
Example of free radical initiators which can be employed in this graft copolymerization reaction include ammonium persulfate, sodium persulfate, potassium persulfate, tert-butylhydroperoxide, hydrogen peroxide and other good hydrogen atom abstracters. Persulfate initiator systems are preferred. Approximately 0.1 to 10 wt % (preferably 0.5 to 3 wt %) of the initiator, based on the amount of total monomer is used.
Weight percent levels of total ethylenically unsaturated monomer relative to poly(vinyl alcohol) typically range from 10 to 90 wt %, preferably from 50 to 90 wt %. Weight percent levels of a crosslinking monomer can range from 0 to 10 wt %; preferably 2 to 6 wt % relative to poly(vinyl alcohol).
The aqueous poly(vinyl alcohol) graft copolymer emulsion binder of this invention can be applied to a variety of filter basestocks or substrates, such as nonwovens, fabric, and cellulosic materials. Filter paper is especially suitable as a substrate; for example, bleached or unbleached filter paper weighing 30 to 180 g/m2.
The binder can be applied to the basestock or substrate in an suitable fashion such as by spraying, dipping, roll transferring, or the like. Application is typically made at room temperature. The solids concentration of the binder is in the range of 10 to 60 wt %, perferably 10 to 35 wt % when applied by dipping. When applied by roll transfer, solids concentration of the binder is generally about 25 wt % whereas, with spraying the solids concentration can vary widely. The amount of binder, on a dry basis, typically ranges from 3 to 50 wt % of the filter substrate.
The impregnated substrate can then be dried and cured by passing it through an air oven or the like. Various time-temperature relationships can be employed for drying and curing as is well known in the art. For example 5 to 10 minutes at 150 to 200xc2x0 F. (66-93xc2x0 C.) for drying and 3 to 5 minutes at 250 to 350xc2x0 F. (121 to 177xc2x0 C.) for curing.
In making a high performance filter paper, the aqueous poly(vinyl alcohol) graft copolymer emulsion binder composition of the present invention can be applied to basepaper by immersing the basepaper in the binder to saturate the paper. After passing the paper through rollers to meter the coat weight, the paper can be dried in an oven at high temperature; for example, 300xc2x0 F. (149xc2x0 C.), for about 8 minutes for a single stage cure. The amount of graft copolymer emulsion (solids basis) added to the basepaper can be 10 to 35 wt %; preferably 20 to 30 wt %.
It is preferred to admix an external crosslinking agent and/or catalyst to the aqueous emulsion binder prior to saturating the paper to promote polymer crosslinking. Suitable crosslinking agents include, glyoxal, glutaraldehyde, and other dialdehydes, colloidal silica, melamine-formaldehyde resins, urea-formaldehyde, zirconium ammonium carbonates, polyamide-epichlorohydrin resins, emulsified epoxy resins, phenol-formaldehyde resins, and polyacrylate resins containing pendant unsaturation and other crosslinking resins. The amount of external crosslinking agent is typically about 1 to 20 wt %; preferably, about 5 to 15 wt %.
Suitable catalysts include ammonium chloride, sodium bisulfate, and other acids to lower the pH of the binder saturant. The amount of catalyst is typically about 1 wt %; however the amount can range from 0.5 to 6 wt %.
The following table (Table 1) sets forth operative and preferred ranges regarding the synthesis and use of poly(vinyl alcohol) graft copolymer emulsions described in this invention.